The present invention relates to environmental control systems and, in particular, to aircraft air conditioning and thermal management.
An aircraft environmental control system typically consists of an engine bleed air-driven high pressure air cycle system which provides conditioned, temperature-controlled, dehumidified air for cockpit and crew member cooling, pressurization, cooling of air-cooled avionics, cooling of liquid-cooled equipment (such as radar) and various other pneumatic utility subsystems. An example of a conventional prior art environmental control system adapted for military fighter aircraft use is depicted in FIG. 1. The air cycle refrigeration system 10 includes an air cycle machine 24, primary heat exchanger (not shown), secondary heat exchanger 13, condenser heat exchanger 15, water extractor 16, reheater 14, liquid-to-air load heat exchanger 18, together with sensors, valves and associated controls (omitted for clarity).
In operation, the compressor 12 driven by turbine 17 compresses preconditioned engine bleed air 11 with the heat of compression subsequently rejected to ambient air through the secondary heat exchanger 13. The cooling turbine 17 portion of the air cycle machine 11 extracts energy from the preconditioned bleed air 11 and chills the air to typically subfreezing temperatures (e.g., xcx9c40 deg F.). This air is delivered to the hot side inlet of the condensing heat exchanger 15 which cools a cross-stream airflow output from the reheater heat exchanger 14, condensing entrained moisture into droplets which are removed in the water extractor 16. This airflow then passes through the reheater 14 and is delivered to the cooling turbine 17 inlet where the air is expanded through the turbine 17, giving up energy and is cooled in the process. This cooled air is further temperature regulated by the addition of hot bypass air through a temperature control valve (not shown) to provide temperature-controlled, dehumidified conditioned air 19.
A complication arises during operation when moisture is present as the air is cooled to below subfreezing temperatures since entrained water present in the air stream condenses into a fine spray of ice crystals. The ice entrained in the air stream will begin to accrete on downstream surfaces and, in particular, on the cold side inlet face of the condensing heat exchanger 15 and, if left unchecked, will back-pressure the turbine 17 and choke off flow.
To prevent excessive ice accumulation, anti-ice provisions, such as internal hot-air hot bars 23 incorporated into the condenser 15 face, are typically employed. Commonly, de-ice provisions are also provided consisting of a hot air bypass or anti-ice valve 22 to allow the hot bleed air 11 to bypass the cooling turbine 17 to melt any accumulated ice once a preset temperature and/or pressure drop is exceeded. The addition of heat to melt accumulated ice substantially reduces the available cooling capacity of the refrigeration unit and it is therefore desirable to minimize the add-heat function to the extent practicable.
Cooling of liquid-cooled loads is typically accomplished as shown in FIG. 1 by means of an added thermal transport loop connected to the refrigeration unit 10 by coolant lines 20 and 21 with suitable pumping means (not shown) to a remotely located liquid-cooled load(s). Waste heat from liquid-cooled load(s) such as radar is rejected to the liquid-to-air load heat exchanger 18 disposed either upstream or downstream of the condenser 15 in a series arrangement. This additional heat often presents a severe performance penalty and must be carefully considered in the design to avoid degrading condenser operation and, hence, water removal and to avoid undercooling of the cockpit or air-cooled equipment.
The series arrangement of condenser 15 in front of the liquid-to-air load heat exchanger 18 as shown in FIG. 1 creates two main performance problems. First, the coldest inlet air the downstream heat exchanger 18 experiences is limited by the minimum outlet temperature of the upstream heat exchanger 15. As a result, the performance of the downstream heat exchanger 18 is often less than optimal or desired, resulting in elevated liquid supply temperatures (e.g., in excess of 110 deg F. for a xcx9c9 kw load in a typical case). For refrigeration packs with the condenser 15 located upstream of the liquid-to-air load heat exchanger 18, the liquid heat load that can be rejected is limited by the maximum air temperature of cooling air delivered to the cockpit and/or air-cooled avionics equipment. In the case of a liquid-to-air load heat exchanger 18 located upstream of the condenser 15, condensing operation may be degraded to an unacceptable degree as a result of high cold side inlet temperatures such that efficient condenser 15 operation no longer occurs. This, in turn, results in excessive humidity and moisture delivered to the cockpit and/or air-cooled equipment and increasing sensible heat.
The second performance problem associated with prior art environmental control systems is inadequate ice control and removal. The prior art approach in high pressure air cycle systems is to reduce the amount of entrained moisture entering the cooling turbine 17 by means of a condenser 15 and a swirl type inertial water extractor 16. The amount of water removed is dependent on the internal surface metal temperature of the condenser 15, i.e., the lower the temperature, the larger the condensed droplets. Not all of the water is necessarily removed, however, particularly under low altitude, moist, tropical day conditions. This entrained moisture condenses into ice crystals as the air is expanded through the cooling turbine 17 to below freezing temperatures. The resultant ice discharged from the cooling turbine 17 tends to accrete on chilled surfaces of downstream ducting and the inlet face of the downstream heat exchanger 15, eventually freezing over the heat exchanger inlet and interrupting airflow unless de-ice or anti-ice control provisions are incorporated. Operation of de-ice or anti-ice controls, however, directly subtracts from the inherent refrigeration capacity of the cooling turbine 17. Use of hot air de-ice should therefore be minimized to avoid excessive conditioned air supply temperatures. A further difficulty arises when ice that is allowed to accumulate and then melt as the resultant slug of liquid water is introduced in the conditioned air stream, necessitating additional drainage provisions.
As may be seen from the foregoing discussion, there is a need for an environmental control system that provides improved efficiency and anti-ice control.
In one aspect of the present invention, a heat exchanger subsystem for an environmental control system comprises a heat exchanger array having a plurality of heat exchanger elements that operate in parallel to an inlet airflow to the heat exchanger array such that each heat exchanger element is thermally connected to a separate load that is thermally independent or isolated from other loads. In particular, an environmental control system is disclosed comprising a heat exchanger subsystem downstream of an air cycle machine cooling turbine, with the subsystem having an air-to-air heat exchanger and a liquid-to-air heat exchanger which operate in parallel with each other when connected to an inlet airflow to the heat exchanger subsystem. The heat exchanger subsystem enables accommodation of relatively large liquid-cooled loads without degradation of cooling or interaction with air-cooled loads.
In another aspect of the invention, an anti-ice control subsystem having an array of parallel heat exchange elements or hot bars is disclosed for use in conjunction with a heat exchanger of relatively narrow coldside air passage fin spacing such as a liquid-air heat exchanger external to and upstream of the liquid-air core. The anti-ice control subsystem enables subfreezing operation of the liquid-air heat exchanger without ice over or blockage of the heat exchanger, particularly in the case wherein integration of conventional internal hot bar heating elements is not feasible.
In a further aspect of the present invention, an environmental control system is described which includes a heat exchanger subsystem for continuous removal of ice crystals and water droplets from an air stream discharged from a cold air source such as an air cycle machine, enabling continuous subfreezing operation of the environmental control system and obviating the need for intermittent de-ice cycling, thereby substantially improving cooling performance. The heat exchanger subsystem comprises a plurality of heat exchange elements in a parallel array for melting of ice crystals present in an air stream impacting the heat exchange elements; a plurality of heated slots or openings disposed between the heat exchange elements for capture of ice crystals or water droplets; a heated circuit or fluid passage(s) connected to a heat source; a cold air circuit or fluid passage(s) connected to a cold air source such as an air cycle machine; a heated chamber, sump, or plenum for collection of water droplets; and heated drainage provisions for disposal of collected liquid water. In a preferred embodiment, the anti-ice heat exchange elements utilize heat energy normally rejected overboard either from hot compressed bleed air or heat recovered from downstream air-cooled or liquid-cooled loads. Ice melted by the recovered heat energy is removed by a heated chamber integrated with the heat exchanger subsystem.
In yet another aspect of the present invention, a method of recovering waste heat from a liquid-cooled load is used to provide anti-ice control by a parallel array of heat exchange elements or hot bars, reducing or eliminating ice accretion within refrigeration system components downstream of the cooling turbine. Recovery of waste heat energy reduces hot bleed air consumption for improved energy efficiency, allows subfreezing cooling turbine operation, and maximizes cooling system performance.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.